Current subjects of considerable research and clinical interest include; 1) the developmental relationships and functional interactions between cells of the skeletal and immune systems, and 2) the effects of 1,25- dihydroxyvitamin D [1,25(OH)2D] on its distinct osteopetrotic mutations in the rat, toothless (tl) and osteopetrosis (op), in which reduced bone resorption coexists with decreased natural killer (NK) cell function and elevated serum 1,25(OH)2D levels to investigate the basis for aberrant natural immunity and vitamin D metabolism and to explore the interrelationships between these extraskeletal abnormalities and reduced bone resorption. The cellular basis for reduced NK cell function will be pursued using a combination of techniques to assess NK cell number, function, morphology and cytochemistry. These include 51Cr-release and single-cell assays, analyzing and sorting NK cells on a fluorescence- activated cell sorter and testing the ability of cytokines to activate NK cells in vitro. The hypothesis that vitamin D resistance is a component in the pathophysiology of osteopetrosis will be tested by assessing 1,25(OH)2D receptor status in several target tissues. Saturation binding and sucrose gradient assays will be performed to evaluate 1,25(OH)2D receptor number and affinity, and receptor mRNA levels will be quantitated by Northern blot and RNase protection analyses in normal and mutant rats of each stock. The susceptibility of op rats to cure and normal rats to induction of osteopetrosis by reciprocal stem-cell transplantation and the resistance of rats of t1 stock to cure/induction by stem-cell and NK cell functions and elevated serum 1,25(OH)2D in osteopetrosis. The clinical significance of the proposed studies is two-fold: 1) in the eventual application of these data to the prevention of localized osteopenias associated with an inflammatory component, such as periodontal disease; and 2) to provide definitive data confirming or refuting the hypothesized resistance to 1,25(OH)2D in osteopetrosis, information which will be important in determining whether the therapeutic manipulation of vitamin D metabolites is possible and/or warranted in the clinical management of some osteopetrotic children. In addition, the data generated by these studies should provide the framework for future investigations concerning immune and endocrine dysfunction in osteopetrosis.